1. Field of the Invention
The present invention relates to optical amplifier equipment used for wavelangth division multiplexing (WDM) optical communication, and more particularly, relates to optical amplifier equipment for WDM optical communication which has a high degree of flatness in a gain of a signal light of each wavelength.
2. Description of Related Art
A WDM optical communication system which wavelength-division-multiplexes a plurality of signal lights with mutually different wavelengths and optically communicates them is widely used, since it can increase the communication capacity. In the WDM optical communication system, optical amplifier equipment is also applied for the purpose of extending the communication distance or the like.
As the optical amplifier equipment, for example, erbium doped fiber optical amplifier equipment (EDFA) is well known, in which an erbium doped optical fiber to which erbium is doped is used as the optical fiber to be an amplifying medium. Optical fiber amplifier equipment is normally configured by a pumping light source which outputs pumping light and a WDM which wavelength-division-multiplexes the pumping light with the signal light and which puts them into an amplifying optical fiber, besides an optical fiber to be an amplifying medium.
As conventional optical amplifier equipment for WDM optical communication, for example, optical fiber amplifier equipment is well known, which is configured by a two-stage optical fiber amplifier using pumping light with a wavelength of the 980 nm band at the front stage and using pumping light with a wavelength of the 1480 nm band at the rear stage for simultaneously attaining low noise and high power. A description will be given by taking such a configuration as an example. In order to finally flatten the gain for each signal light of the wavelength-division-multiplexed signal light (hereafter, referred to simply as "WDM light") outputted from the optical fiber amplifier equipment, a gain equalizer is arranged between a front stage optical fiber amplifier and a rear stage optical fiber amplifier.
The gain equalizer is an optical part having a wavelength characteristic of loss which is the reverse of the wavelength characteristic of gain of the optical fiber amplifier. Therefore, the branched output light is received by a monitor PD, and by controlling a pumping light source so that the output light may be constant, the control is performed so that the gain of the optical fiber amplifier may be kept constant.
However, since the wavelength characteristic of the gain of the optical fiber amplifier changes depending on the gain value, the optimized gain flatness can be established only at one point of a certain gain value. Therefore, there is a probability that the gain flatness may not be kept when the loss of an optical part configuring the optical fiber amplifier changes with the environmental temperature.
That is, even if the wavelength characteristic of the gain of an optical fiber amplifier is flattened through the total of the wavelength band of the signal light at a certain specific gain value, for example, a gain of +10 dB, the gain for light with a short wavelength may relatively be higher than that for light with a long wavelength, or the reverse thereof may occur. In any case, the gain is different depending on the wavelength, and the flatness cannot be kept.
Such a fact does not matter when the optical amplification is performed by a gain at which the flatness is previously compensated, for example, in the case where the optical amplifying control is performed so that the optical output level of the amplified signal light may be kept constant, but it may occur when the optical input level of the inputted signal light is lowered and a higher gain is required for still keeping the optical output level constant.
As one means for avoiding the above described problem, it is also considered to control the optical input level of signal light inputted into the optical fiber amplifier equipment so that the optical output level may be a certain constant value and it may be a gain at which the flatness of the gain of the optical fiber amplifier can be attained.
However, even if the control is performed so that the optical input level of such signal light may be a specified value, it is difficult to keep the flatness of the gain of the optical fiber amplifier, for example, because of the loss fluctuation of the optical part itself configuring the amplifier which is caused by the change of the ambient temperature of the optical fiber amplifier, or various other factors. Finally, it is difficult to keep the flatness of the optical output level of each signal light in the amplified WDM light.